Export c2w, w2c

A downstream dependency https://hackage.haskell.org/package/harfbuzz-pure fails to build if those combinators are removed:

error: builder for '/nix/store/gqsnr3jknkyihkbvqd2lpj42qln0v9rl-harfbuzz-pure-0.1.0.0.drv' failed with exit code 1;
       last 10 log lines:
       >     Module ‘Codec.Binary.UTF8.Light’ does not export ‘w2c’
       >    |
       > 25 | import Codec.Binary.UTF8.Light (encodeUTF8, w2c, c2w)
       >    |                                             ^^^
       >
       > Data/Text/Glyphize/Buffer.hs:25:50: error:
       >     Module ‘Codec.Binary.UTF8.Light’ does not export ‘c2w’
       >    |
       > 25 | import Codec.Binary.UTF8.Light (encodeUTF8, w2c, c2w)
       >    |                                                  ^^^